lantiq: use new image build process for brnImages

LZMA compress the kernel without dictionary, otherwise brnboot fails to
uncompress the kernel.

The filesystem parameter of mkbrncmdline was dropped since the used
filesystem isn't exported in the kernel build step and at least in my
tests everything works without the parameter. If the parameter is
required for some reason, the static keyword "squashfs" can be used.

Signed-off-by: Mathias Kresin <openwrt@kresin.me>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49241 3c298f89-4303-0410-b956-a3cf2f4a3e73